1. Field of the Invention
This invention in general relates to the field of film recorders and, in particular, to a vertical deflection system suitable for use in scanning systems for film recorders.
2. Description of the Prior Art
Scanning-type film recorders and printers produce an image by printing a line of image data at a time, building the image by drawing many sequential horizontal lines. A well-known method of printing on film, for example, uses a cathode ray tube (CRT) as the imaging light source. Positioning the CRT beam in the vertical dimension is commonly done with a digital-to-analog converter (DAC). Monolithic DACs typically have a maximum resolution of 16 bits. With 16 bits available to position the CRT beam in the vertical dimension, there are available 65,536 possible locations available for beam placement. In a high-resolution film recorder there can be as many as 6372 scan lines in an image (i.e., 8 k horizontal resolution with 4".times.5" film). This means that if a single DAC is used for positioning the beam in the vertical dimension, there will be no more than a 10 least-significant-bit change from line-to-line of beam position values.
When creating a flat field image (e.g., a single color gray image) it has been found that line-to-line position errors need to be less than 5% of the line-to-line distance in order that individual scan lines not be perceived as horizontal lines in the image. With a 16-bit DAC system there are at least two sources of errors. One source is the rounding off of data in the generation of the vertical deflection data. This occurs because film recorders accept images of any horizontal resolution from 512 to 8192, and the line-to-line distance may not be an integer value of 16 bits. There may be rounding-off errors as large as one least significant bit (LSB).
The second error type comprises differential non-linearity errors in the DAC transfer function of at least one LSB. A one LSB error will give a line-to-line error of about 10%, which results in visually-perceived horizontal lines in the image. Thus, a vertical deflection system with a line-to-line resolution of greater than 16 bits is necessary to create a flat field image that is free from horizontal line artifacts. At a vertical resolution of 6372, 19 bits of resolution will give a line-to-line error of 1.2% for a one LSB error.